Utility poles and railroad cross ties are normally pressure treated with toxic preservative compositions such as creosote or pentachlorophenol dissolved in oil. These toxic ingredients penetrate thru most of the sapwood portion of the article, but seldom penetrate the heartwood. These pressure treated commodities last up to 30 years without supplementary treatment, but there is frequently a small percentage of early failures.
Supplementary or remedial treatments can be used to materially increase the useful life of poles and ties. Poles show early failure due to light or shallow treatment, failure to destroy internal decay organisms present in the article at the time of pressure treatment, checking or cracking which allows entry of either decay or insects into untreated heartwood, and/or leaching of the initial preservative from the groundline area. Ties most often are attacked under the tie plate due to entry of deterioration organisms into the untreated heartwood thru the spike holes and because of moist conditions under the tie plate.
Most remedial treatments are made by external application. Successful preservation requires 1/16" to 1/2" application of the preservative to the wood surface. Paste or grease type compositions have been developed which adhere to the wood in these thicknesses. These compositions must release the preservative actives into the wood to be effective, and these actives must quickly penetrate to the site of infection to prevent wood strength loss. One or more of the preservative actives should fix in the wood to discourage future infection.
A good determination of whether a remedial preservative is releasing its active ingredients into the wood can be made by taking borings from the wood and checking for chemical penetration. The release of actives into the wood should be swift; otherwise leaching and erosion may remove the preservative from the wood surface and flush much of it into the surrounding soil.
Most external remedial treatments are made to poles at the groundline by brushing on the preservative and covering the treated area with a plastic or coated paper wrap. External remedial treatments are made to ties by application under the plate. Poles are internally treated by inserting a paste, rod, or liquid in holes drilled as close as possible to areas of suspected internal deterioration.
One researcher has described remedial preservatives as those which often include two or more active ingredients which differ substantially in capability for diffusion thru wood. One component is usually quite mobile whereas the other is less or slightly mobile. Historically, remedial preservatives have contained oil based preservatives such as pentachlorophenol or creosote as the slightly mobile component. Copper naphthenate has recently found favor in replacing the more hazardous pentachlorophenol and creosote. U.S. Pat. No. 4,661,157 teaches the use of shaped alkaline earth metal borates for remedial treatment; however, these chemicals alone are not sufficiently permanent in the wood to provide long term protection for poles and ties. Sodium fluoride is also a well known mobile component of wood preservatives.
U.S. Pat. No. 5,078,912 discloses the use of amine-inorganic copper complexes for preserving solid wood when said complexes are diluted from about 1 to 5 percent in water and applied by pressure treatment. No one has proposed amine-inorganic copper complexes for use as the less mobile component in remedial wood preservative compositions. The most compelling reason why these complexes have not been considered is this: remedial preservative components which fix and are most permanent in the wood have always been oil based. No one skilled in the art has proceeded beyond this conventional reasoning.
Conventional thinking also indicates that a water soluble component, such as amine-inorganic copper complexes, will not penetrate poles and ties previously treated with creosote or pentachlorophenol in oil. It is also known that alkaline copper complexes are neutralized by wood acids. The ones which fix in the wood become insoluble upon neutralization, and resist further penetration as effectively as they resist leaching. Also, no economical technology for the thickening of amine-inorganic complexes has been proposed. This is another reason why they have not been considered for use in remedial wood preservatives.